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A suspicious bushfire burning in Western Australia’s Great Southern region has left a blanket of smoke across the southern part of the state, including Perth.

The uncontained blaze began at 4pm (WST) on Friday in bushland in Mount Roe National Park, about 30 kilometres northwest of Denmark.

The cause of the fire is “suspected deliberate lighting”.

A watch and act alert remains in place for people in the western part of the Shire of Plantagenet, east of Clear Hills Road, south of Muir Highway and west of Pile and Springs roads.

“There remains a possible threat to lives and homes, as a fire in the area is still burning and conditions are changing,” the Department of Parks and Wildlife (DPaW) said.

The bushfire is moving at a moderate speed in a northwesterly direction and has burnt through 13,300 hectares.

DPaW warns burning embers are likely to be blown into neighbouring properties and bushland.

About 100 firefighters are using 36 appliances and six heavy machines to bring the fire under control.

DPaW has also issued a smoke alert for areas from Walpole to Geraldton and inland to Toodyay and Mount Barker.

The smoke is expected to clear on Monday.

“If the smoke is particularly thick, it is recommended people shut doors and windows and turn off air-conditioners,” DPaW said.

“People with asthma and pre-existing respiratory, cardiovascular illnesses or diabetes should follow their pre-prepared treatment plan.”

Motorists have also been warned to take care if their visibility is hindered and to turn on headlights.

Suspicious behaviour should be reported to Crime Stoppers on 1800 333 000.